Optical coherence tomography (OCT) is a medical imaging technique for non-invasive imaging based on low-coherence interferometry employing near-infrared light. The OCT device produces three-dimensional (3D) images, with a resolution typically of a few microns, and is widely used in ophthalmology due to the translucent nature of the human eye and the ability to resolve details in the structure of the retina that are important in eye pathology.
Spectral-domain OCT (SD-OCT) is a form of OCT in which the interferometric signal between a reference beam and the back-scattered component of a sample (probe) beam directed into the retina is split into its frequency components by a dispersive device and collected by a line camera. The collected data contains the spectral information of the backscattered signal. This spectral data can be transformed to the spatial domain to obtain a one-dimensional (1D) spatial distribution, referred to as an A-scan, representative of the scattering properties of the sample. Scanning the sample beam across the retina, produces a series of adjacent A-scans which can then be used to create a two-dimensional (2D) tomogram, called a B-scan. A volume representation can be acquired by further scanning the sample beam in a third direction to collect a series of B-scans that covers the three-dimensional (3D) volume of interest.
The OCT signal measures the reflectivity and absorption of the biological tissues. However, determining the functional properties of such tissues is as important as the structure revealed by the backscattered intensity. In recent years, functional extensions of OCT have been investigated that can provide further information. For example, Doppler OCT can provide information about blood flow, and Polarization-Sensitive OCT (PS-OCT) can provide information about the polarization properties of the tissues.
Polarization-Sensitive OCT provides information about the polarization properties of turbid media. The eye has many structures that have useful polarization properties that have been the subject of PS-OCT research, for example the birefringence of the retinal nerve fiber layer (RNFL) has been suggested to be linked to the microtubule density. In addition to birefringence, polarization-preserving and polarization-scrambling structures can be distinguished by PS-OCT. This property has been used to identify the retinal pigment epithelium (RPE) tissue layer in the retina by identifying the polarization scrambling layer located near the RPE. The RPE is of significant importance to vision and damage or distortion of the RPE is associated with age-related macular degeneration (AMD) and progressive loss of vision.
Polarization scrambling tissues in the retina, such as the RPE, cause back-scattered light to be depolarized. Measuring the degree of such depolarization is an important function of a PS-OCT device when identification of RPE tissue is required. Measuring the depolarization, typically requires receiving multiple estimates of the polarization state of back-scattered light from the same location in the retina at different times. The degree to which these measurements differ can be used as a measure of the depolarization of the light.
However, in an OCT device it is currently difficult to record multiple measurements at the same location when measuring a volume. This is due to the limited speed of OCT capture and to the motion of the retina while capture is taking place. This leads to a choice of sampling the same point multiple times in a fixed time by limiting the resolution of the captured volume, or taking a longer time to sample the volume at the same resolution leading to greater distortion in the sampled volume. Therefore, it is preferable to have a method of determining the degree of polarization in a PS-OCT volume scan without repeated measurement of the same location.
In recent years, a measure of the depolarization of back-scattered light in a PS-OCT device has been demonstrated by extending the concept of the degree of polarization to a spatial region. This is called the degree of polarization uniformity, DOPU, and measures the degree of spatial change in the back-scattered polarization signal. However, DOPU can only be calculated in regions that have a high intensity, and the size of the spatial region used to calculate the DOPU causes a change in the size of the region that appears to be depolarizing (i.e. polarization scrambling).
Therefore, there is still a need for a method for measuring the depolarization of regions within OCT scans that overcomes these problems.